A number of foods, drinks, chemicals, etc., are susceptible to deterioration, putrefaction, denaturation, or the like when brought into contact with the oxygen in air. A number of methods have been practiced in the past for preventing this from occurring. For example, deoxidants have been sealed in containers. Such deoxidants can comprise reductive organic compounds, for example, sulfoxilates, dithionates, or the like. The deoxidants are capable of absorbing oxygen. They can be put in a sealed container or wrapped with a permeable film, together with the foods, drinks, chemical, and the like.
For sealed containers such as food cans, beer cans, juice cans, and the like, the oxygen in the air in the head space or dissolved in the contents of the sealed containers can be replaced with inert gases such as nitrogen or carbon dioxide.
One specific attack on the problem of oxygen leading to deterioration of foods is discussed in European Patent Application 301 719 published Feb. 1, 1989. The aforementioned patent application discloses a wall for a package, which wall comprises or includes a layer comprising a composition comprising a polymer and having oxygen-scavenging properties. The composition scavenges oxygen through the metal-catalyzed oxidation of an oxidizable organic component. The oxidizable organic component is preferably itself a polymer and may be the only polymer in the composition. The preferred composition taught in the aforementioned patent application is a blend of 96% polyethylene terephthalate (PET) and 4% poly (m-xylyleneadipamide) containing 200 ppm cobalt as a catalyst. Rhodium and copper are also taught as being alternative catalysts. Polyamides such as MXD6 are taught as being oxidizable polymers which are of particular interest for use in the composition and process of the aforementioned patent application. The PET is taught as being useful in controlling the permeability to oxygen of the composition. While most of the examples and discussion in the patent application is concerned with the use of PET it should be noted that Examples 10-12 utilize polypropylene (PP) or polyethylene (PE) instead of PET.
Another European Patent Application of interest is Application No. 335 520 published on Oct. 4, 1989. This application incorporates the teaching of European Patent Application 301 719 and utilizes the composition of that application in a sandwich form between other layers, which other layers may comprise a maleic anhydride modified polypropylene. However, there is no teaching of blending of a maleic anhydride modified polypropylene within oxidizable polymer such as a polyamide or with a metal salt catalyst.
Another recent publication of interest is International Publication No. WO 90/00504 which was published on Jan. 25, 1990. This publication discloses a polymer composition for manufacturing containers, intermediate forms or part thereof for improving their oxygen barrier properties. The composition contains as an active component a metal compound having the capacity of scavenging oxygen and consisting of a metal ion having complexing properties and a polymer to which the metal ion is combined as a complex. The polymer composition, in admixture with a further polymer, can be molded into containers, intermediate forms or parts thereof. This publication discloses the use of mixtures of PET and a polyamide for producing plastic materials by injection molding. If such materials also include a metal which complexes with ligands originating from groups and/or atoms present in the polymer, a scavenger effect for oxygen is attained. Listed metals include iron, cobalt, nickel ruthenium, rhodium, palladium, osmium, iridium and platinum. Copper, manganese and zinc are also listed. In addition to the use of PET-polyamide mixtures the use of polyethylene, polypropylene, polyvinylchloride and their copolymers is mentioned. International Publication No. WO 90/00578 includes a similar disclosure.
None of the above-mentioned publications reports a problem which I have discovered, namely, that if containers are prepared from polyolefins, by which is meant polymers wherein the polymer backbone contains substantially only --C--C-- units, rather than from PET, for example, from polyethylene, polypropylene, polybutylene or copolymers thereof, or an ethylene copolymer with vinylacetate, with an acrylate such as methyl, ethyl or butyl acrylate, or with an alkyl acrylate such as methylmethacrylate, or mixtures of two or more of the above polymers, and an oxidizable polymer, for example, a polyamide such as MXD6, along with a metal salt, for example a cobalt salt, much poorer oxygen-scavenging is obtained than if PET is used rather than the polyolefin. I have also discovered by utilizing scanning electron microscopy (SEM) that polyolefin-MXD6-cobalt salt blends prepared by melt mixing are inhomogeneous and phase-separated. I believe that this is the main reason why such blends are relatively poor oxygen scavengers when compared to PET-MXD6-cobalt salt blends. The present invention is directed to overcoming the problem as set forth above.